The present invention relates generally to contact lenses and to methods of manufacturing contact lenses, and in particular to asymmetric, aspheric contact lenses and methods of manufacture thereof.
Thirty to forty percent of the human population under age 40 develop an ocular refractive error requiring correction by glasses, contact lenses, or surgical means. Refractive errors result when the primary optical elements of the eye, the cornea and the lens, fail to image incoming light directly on the retina. If the image is focused in front of the retina, myopia (nearsightedness) exists. If the eye image is focused behind the retina, hyperopia (farsightedness) exists. The focusing power of the eye or any of the eye""s individual components is measured in units called diopters.
Approximately 20% of the patients under 40 having vision defects cannot wear contact lenses because the contact lenses do not fit (become dislodged and/or are very uncomfortable), or they fail to provide the requisite optical correction, or both. In addition, many patients who currently wear contact lenses are not satisfied with the length of time they can wear their lenses and/or with the visual acuity their contact lenses provide.
Over age 40, the percentage of the population requiring vision correction dramatically increases because the crystalline lens of the eye becomes relatively inelastic. The quality of the tear film decreases and the problems encountered with existing contact lenses become much more common and acute.
Standard contact lenses are rotationally symmetrical and spherical, vault from the sclera and rest on the cornea. The human cornea, however, is an xe2x80x9casymmetrically asphericxe2x80x9d surface. xe2x80x9cAsphericxe2x80x9d means that the radius of curvature along a corneal xe2x80x9cmeridianxe2x80x9d is not a constant (a xe2x80x9cmeridianxe2x80x9d could be thought of as the curve formed by the intersection of the corneal surface and a plane containing the pupillary axis). Indeed, the corneal curvature tends to flatten progressively from the geometric center to the periphery. xe2x80x9cAsymmetricxe2x80x9d means that the corneal meridians do not exhibit symmetry about their centers. The degree to which the cornea is aspheric and/or asymmetrical varies from patient to patient and within the same person.
Conventional corneal topology mapping systems, such as the PAR CTS,locate the line-of-sight and then map the corneal surface topology in two-dimensional Cartesian space, i.e., along x- and y- coordinates perpendicular to the xe2x80x9cline-of-sight.xe2x80x9d The resulting topology map can be used by the practitioner to plan the surgical procedures. The xe2x80x9cline-of-sightxe2x80x9d is a straight line segment from a fixation point to the center of the entrance pupil. As described more fully in Mandell, xe2x80x9cLocating the Corneal Sighting Center From Videokeratography,xe2x80x9d J. Refractive Surgery, V. 11, pp. 253-259 (July/August 1995), a light ray which is directed toward a point on the entrance pupil from a point of fixation will be refracted by the cornea and aqueous and pass through a corresponding point on the real pupil to eventually reach the retina.
The present inventor has discovered that the cornea, in most patients, is actually naturally tilted to a varying degree with respect to the pupillary axis and the line-of-sight of the eye of the eye. Additionally, the degree of corneal tilt varies within the individual cornea depending on the diameter over which the tilt is measured. More specifically, the intersection between the cornea and the sclera (i.e., the base of the cornea) is tilted with respect to a reference plane that is perpendicular to the line of sight. Research with actual patients has also demonstrated that the eye lids, over time, cause distortions in the shape of the cornea. Specifically, the upper lid causes flattening at the top of the cornea, while the lower lid causes a depression at the bottom.
Spherical lenses do not match the corneal curvature and geometry, and therefore do not fit properly. Furthermore, they do not take into account the corneal tilt or distortion discovered by the inventor. The more irregular the patient""s cornea the worse the fit, such that about 20% of the patients under age 40 are unable to wear standard contact lenses.
So called xe2x80x9csoftxe2x80x9d contact lenses have alleviated some of the problems that patients have experienced in not being able to wear contact lenses or in not being able to wear them for sufficiently long periods of time. This is due, not only, to their relatively soft surfaces, but also to their pliability, which permits them to modify their shape somewhat with different eyes. However, it is this pliability which permits the lenses to flex, in an effort to conform more closely to the underlying corneal shape, and this flexing results in a change in the power of the lens.
The present inventor has found that the portion of a soft contact lens which rests on the sclera of the eye has little effect on lens distortion, owing to the malleability of the conjunctiva, the clear material which covers the sclera (white of the eye). Instead, the distortion is caused primarily by an outer region of the cornea proximate to the sclera.
It is an object of the invention to provide a contact lens that accounts for the natural tilt and distortion of the cornea.
It is another object of the invention to rapidly and economically manufacture contact lenses that provide increased visual acuity by aspherically and asymmetrically matching and/or conforming to a portion of the wearer""s cornea.
These goals are achieved by using information obtained by surface modeling the cornea, and by manipulating this information to design a contact lens that takes into account the local geometry of the cornea, including corneal tilt and distortion. In particular, a contact lens is designed in which includes a central optical portion and a transition portion radially outward of the central portion and overlying the cornea, which is conformed to the local geometry of the underlying portion of the cornea, including corneal tilt and distortion. In a soft contact lens, a scleral skirt is also provided and the transition portion connects the central optical portion and the scleral skirt. In an alternate embodiment representing a less expensive lens, the transition portion is designed to conform to a statistically average cornea. The central optical portion may have a posterior surface which is customized, is toric or spherical, depending on whether a custom or inexpensive soft lens is being designed.